1. Field of the Invention
The present invention relates to a Gm-C filter of a semi-conductor component, and more particularly to an automatic tuning Gm-C filter of a Gm-C time constant tuning circuit of a semi-conductor component.
2. Description of the Prior Art
A filter is required in most communication transmission application circuits. In general, a discrete-time filter can control a bandwidth more accurately, but the discrete-time filter is preferably more suited for a narrow bandwidth. A high frequency broadband circuit generally utilizes a continuous-time filter; and for the high frequency broadband circuit, a Gm-C filter will be a better choice, as its power consumption is low. The most common problems found in the Gm-C filter is that there is always a variation of 30% to 50% during the manufacturing of circuit components in an integrated circuit, and the variation of a capacitance is approximately 10% to 20%. The degree of variation causes a slight difference in the actual value and the preset value of a filter of Gm-C time constant. As a result, the accuracy of the bandwidth of the filter is affected. Therefore, this approach is disadvantageous for a circuit that requires accurate frequency response. In addition, when the circuit is operating, the Gm-C time constant changes, according to temperature, voltage, and so on. These environmental changes cause a great offset to be generated such that the frequency response of the Gm-C filter cannot be amended immediately and the efficiency and accuracy of the circuit is therefore further affected. In a typical modern technology environment, the semi-conductor component that utilizes the Gm-C filter is generally coupled with an additional auto tuning circuit. The additional auto tuning circuit is required to overcome the problem of cut off frequency offset due to the changes in the operating environment.
In practice, there are two most applicable methods utilized to tune the Gm-C time constant of the filter. The first common method is to utilize a phase lock loop (PLL) to lock a frequency, wherein a portion or a whole of a VCO that collocates with the PLL is copied to tune the Gm-C time constant of the filter. Additionally, feedback signals from other circuits are collocated to control the conductance value of the Gm-C time constant of the filter. For example, John M. Khoury published an article on “Design of a 15-MHz CMOS Continuous-Time Filter with On-chip Tuning” in the IEEE Journal of Solid-state Circuit. In this article, Khoury discloses a five-level filter that utilizes PLL to adjust cut off frequency. However, the main disadvantage present in Khoury's five-level filter is that this type of circuit has a great surface area and therefore its power consumption is high; also the completion time taken for tuning is long and the tuning speed can be very slow as the Gm-C time constant can only be adjusted after the PLL is settled.
The second conventional method of tuning is to copy a portion of conductance and capacitance of the Gm-C time constant of the filter utilizing a method of integration to obtain a ratio of conductance to capacitance and collocating feedback signals from other circuits to control the conductance value. For example, luri Mehr and David R. Welland published an article on “A CMOS Continuous-Time Gm-C Filter for PRML Read Channel Application at 150 Mb/s and Beyond” in the IEEE Journal of Solid-state Circuit, they teach the utilization of an integrated circuit to obtain a plurality of ratios of conductance to capacitance similar to the ratio of conductance to capacitance of the variation filter such that the cut off frequency of the filter can be adjusted.
Please refer to FIG. 1. FIG. 1 illustrates a diagram of a conventional Gm-C filter of a Gm-C time constant tuning circuit 100. A reference conductor 110 is a copied portion of a conductance Gm of the filter of the Gm-C time constant being tuned by the tuning circuit 100. It represents that the reference conductor 110 and the conductance Gm of the initial filter has the same characteristics, which causes the same degree in variation in manufacturing. A reference capacitor 120 is a copied portion a capacitance C of the filter of the Gm-C time constant being tuned by the tuning circuit 100. It represents that the reference capacitor 120 and the capacitance C of the initial filter has the same characteristics, which causes the same degree in variation in manufacturing. As shown in FIG. 1, the reference conductor 110 receives a reference voltage Vref and the output terminal coupled to the reference capacitor 120 to form a time variable voltage signal Vs1(t). The output terminal of the reference conductor 110 then couples to the two switches 132 and 134 according to a clock signal clk1 coupled to negative terminals of two comparators 142 and 144. The positive terminals of the comparators 142 and 144 receive two reference voltages Vtp, Vtm, the output terminal then connects to a digital logic unit 150. The digital logic unit 150 outputs two signals Up, Dn according to outputs from the comparators 142, 144 and a second clock signal clk2. The signals Up and Dn are outputted to a charged pump 160. The charged pump then outputs a current lcp1. After the current lcp1 flows through a filter 170, a control voltage Vcon1 is feedback to the reference conductor 110 to control the conductance value of the reference conductor 110. Wherein the conductance value of the reference conductor 110 is represented as gm, and the capacitance of the capacitor 120 is represented as c1, thus the formula of time variable signal Vs1(t) is as shown below:Vs1(t)=Vref.gm/c1.t  Formula(1)
Please refer to FIG. 2. FIG. 2 illustrates a variable graph indicating the voltage signal Vs1(t), Up1, and Dn1 of FIG. 1 with respect to time wherein the reference voltages Vtp and Vtm are positioned at horizontal dotted lines with the time variable voltage signal Vs1(t), and Tclk1 and Tclk2 are cycle lengths of the clock signals clk1, clk2 respectively, and Tdn1 and Tup1 represent high voltage time periods for the voltage signals Up1 and Dn1 respectively. In coordinating with FIG. 2, the feedback of FIG. 1 must be stable and the length of the voltage signal Up1 and Dn1 are also required to be equal, which means that the length of Tdn1 and Tup1 are to be equal as well. It can be concluded from FIG. 1 and FIG. 2 that:Vs1(4.Tclk2)=(Vtm+Vtp)/2  Formula(2)
In combining formula 1 and formula 2 the following formula can be obtained:Vref.gm/c1.(4.Tclk2)=(Vtm+Vtp)/2  Formula(3)
The ratio gm/cl of the reference conductor 110 and the reference capacitor 120 can be determined through formula (3). Thus the Gm-C time constant Gm/C of the filter of the Gm-C time constant being tuned by the tuning circuit 100 can be reintroduced. When the Gm-C time constant Gm/C of the filter of the Gm-C time constant being tuned is known, then the methods such as adjusting the control voltage, can be utilized to adjust the conductance value of the filter of the Gm-C time constant being tuned to correct the Gm-C time constant.
However, although the resulting tuning speed of the conventional integration method of FIG. 1 for tuning the Gm-C time constant of the tuning circuit 100 is much faster than utilizing the PLL to tune a circuit, there remain many inevitable defects still present given this approach. Firstly, although the reference conductor 110 and the reference capacitor 120 each are only required to copy a portion of the original conductance and capacitor, the latter process is still complicated. In addition, the two comparators 142, 144 take up a large portion of surface area in the circuit. Moreover, as the charged pump charges and discharges electricity on the filter 170 according to the voltage signal Up1 and Dn1, according to FIG. 2, the voltage signals Up1 and Dn1 are two non-overlapping voltage signals, which means that even if the tuning circuit 100 is stable, the charged pump 160 still continues to charge and discharge electricity on the filter 170; therefore the control voltage Vcon1 being feedback to the reference conductor 110 by the filter 170 is still in vibration and is not a fixed value. Lastly, the variation in manufacturing will also affect other parameters such as the accuracy of the reference voltages Vtm, Vtp; these will affect the conductance and capacitance constant of the filter being tuned according to the ratio of the reference conductor 110 to the reference capacitor 120 which also affects the efficiency of the frequency response of the Gm-C filter and the entire circuit.